added display and its subclasses

src/cursor.cc

@@ -17,6 +17,10 @@ void cursor::show() const {
         refresh();
 }
 
+void cursor::clear() const {
+        ::clear();
+}
+
 void cursor::print_char(const position &pos, const char ch,
                         const int attrs) const {
         attrset(attrs);